Pelletizing device for cutting plastic fibre strands into pellets, having a strand-feeding device and a cutting rotor, according to the preamble of claim 1.
Pelletizing devices of this nature are used in strand-pelletizing installations, in which a plastic material in the molten state is extruded from dies to form strands, which are initially cooled in a feed channel until their surface is no longer sticky and are then fed to a pelletizing device by means of a strand-feeding device. A cylindrical cutting rotor, which is equipped with cutting blades on its circumferential surface and cuts the plastic fibre strands into pellets, rotates at a high speed in the pelletizing device. Pellets of this nature can then be provided in the form of plastic granules for further processing steps.
For this reason, the cutting rotor, together with its blades, forms the principal component of a pelletizing device of this nature. These blades are distributed over the circumference of the rotor and can be clamped with their root regions in grooves in the rotor body by mechanical, pneumatic, hydro-mechanical or electromechanical means. Naturally, it is also conceivable to use cutting rotors in which blades, which are preferably made from sintered carbide, are soldered into the grooves of the rotor body, but solutions of this nature have the drawback that the blades cannot be exchanged without considerable effort.
The publication EP 0,357,549 A1 has disclosed a cutting rotor with blades which are clamped in grooves in a rotor body, the blades having at least one concave clamping surface, against which a conical screw bears, the axis of which is parallel to the longitudinal direction of the blade, the cone of the conical screw bearing eccentrically against the clamping surface. This known solution provides two conical screws which are clamped against one another for one blade, one conical screw securing the blade from one edge of the blade in the axial direction, and the second conical screw securing the blade from the opposite edge thereof. In this way, the blade is fixed and preloaded in the axial direction on the rotor circumference.
This known solution has the drawback that pressure is exerted on the blade from both sides, and thus the blade can be bent or distorted adversely in the tolerance range of the groove in the rotor body. This, on the one hand, entails the drawback of geometric inaccuracy and, on the other hand, may also cause unforeseen imbalance.
A further drawback of the axial clamping in the known solution is that the conical screws are subjected to tensile loads at their critical transition from the cone to the screw thread, so that the notching effect of the transition from the screw thread to the cone may cause the entire cone, together with the screw head, to tear off due to notch cracking. In the event of such damage, for example, the carbide cutter is no longer held in place by anything, since only the screw thread of the conical screw remains in the rotor body, so that, for example, the sintered carbide blade is thrown out of the groove by the centrifugal force of the rotor, thus endangering the entire pelletizing device. To avoid notch fractures of this nature, the conical screw has to be oversized and subjected to special machining steps during its manufacture, in order to minimize the notching effect at the transition from the screw thread to the conical area. Furthermore, a complete threaded hole has to be incorporated in the material of the rotor body, in order to ensure that the conical screws can be clamped securely in place. A large amount of the rotor-body material is also required for these holes, resulting in the further disadvantage that it is impossible to distribute as many as blades as required over the rotor circumference, since this mechanical form of attachment using conical screws requires a considerable volume between two blades.
The object of the invention is to provide a pelletizing device according to the preamble of claim 1 which overcomes the drawbacks of the prior art and, in particular, results in a cutting rotor which allows the maximum possible number of cutting blades to be positioned on the rotor circumference and reduces the risk of a cutting blade being thrown out, as well as avoiding distortion of the blades.
This object is achieved by means of the features of the present invention.
To this end, there is provision for the pelletizing device for cutting plastic fibre strands into pellets to be equipped with a strand-feeding device and a cutting rotor, which is rotated by a drive system and provides cutting blades distributed over its circumference. The cutting blades are positioned in grooves. Parallel to its cutting edge, each cutting blade has at least one recess which, in the installed state, is covered by the groove. A clamping element which is supported against the rotor body engages into the recess in the cutting blade and ensures that the cutting blade is held in position. According to the invention, the clamping element comprises a slotted clamping sleeve, the cylindrical outer contour of which is of the same shape as the recess in the cutting blade.
The solution according to the invention has the advantage of using a thread-free clamping element in order to hold the blade in position on the rotor circumference. The smooth surface of the slotted clamping sleeve is not exposed to any notching effects whatsoever. Since the cutting blades are subjected to essentially centrifugal forces, they can be held securely by the clamping sleeves, which engage in a form-fitting manner into the recess in the cutting blades.
The preloading of the slotted clamping sleeves makes it possible to compensate for the slight axial forces which could cause axial displacement of the blades by means of the force-fitting connection between the preloaded, slotted clamping sleeve and the cutters. Thus, the solution according to the invention ensures that it is impossible for the blades either to be displaced in the axial direction or to break out in the radial direction. In this solution, there is no possibility whatsoever of a conical extension breaking off from a threaded area of a conical screw.
Prior to installation, the diameter of the unclamped, slotted clamping sleeve is greater than the diameter of the recesses which are provided in the rotor body and on the cutter surface, parallel to the cutter edge, for a clamping sleeve of this nature.
Preferably, a clamping sleeve of this nature may extend over the entire width b of a blade. For this purpose, a corresponding recess is to be provided over the entire width b of a blade. However, it is also possible to provide recesses on each side of the blade, which recesses engage with two corresponding clamping sleeves which are supported against the rotor body.
A continuous clamping sleeve has the advantage that it can be fitted and removed easily by being pressed in the axial direction into the recesses provided in the rotor body and in the blade and due to the fact that it can be pressed out with the aid of a mandrel in the event of removal being necessary. In the case of recesses which do not extend over the entire width, two clamping sleeves are required on either side. For these clamping sleeves to be removed, they preferably have an internal screw thread into which withdrawal means can be screwed.
In a preferred embodiment, the recess in the cutting blade and an opposite recess in the rotor body together form a cross section with segments of a circle which are offset with respect to one another. This offset of the circle-segment cross sections of the recesses with respect to one another is minimal and ensures that there is not only a clamping action in the tangential direction on the cutting blade from the clamping sleeve, but also a radial component pressing the blade onto the rotor. For this purpose, the circle-segment cross section of the recess in the cutting blade is preferably offset radially outwards with respect to the circle-segment cross section of the recess in the rotor body. An offset in this direction causes the clamping sleeve to press the blade radially into the groove of the rotor body, so that it is impossible for there to be any play between the base of the groove and the cutting blade.
The clamping sleeve is preferably made from spring steel, in order to provide the necessary preloading by deformation.
In a preferred embodiment of the invention, the clamping sleeve has an internal screw thread which extends inwards from the outer edge of the clamping sleeve and merges into a smooth, inwardly tapering inner cone in the end region of the clamping sleeve. On the one hand, the internal screw thread makes a clamping sleeve of this nature easy to remove, and on the other hand the clamping sleeve, due to a smooth, inwardly tapering inner cone, has a greater spring action in the inner area than in the outer area.
In a further preferred embodiment of the invention, the inner cone of the clamping sleeve, by the insertion of a grub screw, can be brought into engagement with an attached cone. In this case, the grub screw, in the area of the inner cone of the slotted clamping sleeve, will widen the latter in the segmental direction with respect to the cutting rotor, towards the recess in the cutting blade. The widening of the clamping sleeve, when the grub screw is screwed in, connects the cutting blade to the rotor body in a form-fitting and force-fitting manner by means of the clamping sleeve.
In contrast to the prior art cited above, the grub screw does not have to be oversized, but rather can be kept as small as possible, since the transition between the inner cone of the grub screw and the screw thread of the grub screw is not subjected to tensile loads, but rather to compressive loads. This makes notch fractures virtually impossible. Even if a notch fracture of this nature were to occur, the cone, which presses the clamping sleeve against the recess in the cutting blade, could not pop out, as happens in the prior art, and thus release the cutting blade, but rather remains in the inner area of the clamping sleeve and is held in place by the threaded area of the grub screw. A grub screw can be screwed into and out of a clamping sleeve of this nature by means of a hexagon socket in the threaded area of the grub screw.
In a further preferred embodiment of the invention, the clamping sleeve has an internal screw thread which tapers conically inwards from the outer edge of the clamping sleeve. A simple cylindrical grub screw can be accommodated by the screw thread and, when screwed into the clamping sleeve, widens the latter in the segmental direction with respect to the cutting rotor, towards the recess in the cutting blade. The cutting blade is then connected to the rotor body in a form-fitting and force-fitting manner by means of the clamping sleeve, the form fit acting essentially in the radial direction, and the force fit prevails in the axial direction.
This solution has the advantage that only the slotted clamping sleeve has to have a tapering internal screw thread, while a cylindrical, standardized grub screw can be used for prestressing of the clamping sleeve. A further advantage of this solution is that in this case too the grub screw can be subjected to entirely compressive loads, so that there is virtually no possibility of a notch fracture in the area of the screw thread.
Preferably, an eccentric extension is arranged on the inner end of the clamping sleeve. This eccentric extension may be designed as a cylindrical elongation which is parallel and offset with respect to the axis of the clamping sleeve. When the cutting blades are being fitted into the cutting rotor, the eccentric extension advantageously prevents the clamping sleeve from turning, so that the grub screw can be screwed reliably into the clamping sleeve during assembly. To this end, the eccentric extension is advantageously supported on that area of the cutting blade which is free from the recess or is advantageously in engagement with a correspondingly offset hole in the rotor body as an extension of the recess 12 in the rotor body.
A preferred material for the clamping sleeve is spring bronze. This material is not only highly elastic, in the same manner as spring steel, but also allows limited plastic deformation, so that the spring bronze can be adapted more easily to shifts and offsets in cross section between the recess in the cutting blade and the recess in the rotor body.
Preferably, the cutting blades, with their root area in the grooves, are arranged in the rotor body with a uniform distribution on the rotor circumference, at an acute angle, in the axial direction, of less than 10 degrees, preferably between 3 and 6 degrees, with respect to the rotor axis of the cutting rotor. This arrangement has the advantage that the pellets are not knocked off the plastic fibre strands, but rather can be cut off, in the pelletizing device.
In a preferred embodiment of the invention, the recess in the cutting blade is formed on the cutting-edge side. With this preferred arrangement of the recess, the cutting blade is pressed, by the clamping element, i.e. the clamping sleeve, onto that surface of the groove in the rotor body which is opposite the cutting edge. In this way, the blade is supported over a large area by the rear wall of the groove, since the clamping element presses the entire surface of the blade against the rear wall of the groove. Naturally, the clamping element and the recess may be made on the other side, which lies opposite the cutting edge, but in this case the bearing surfaces are considerably reduced in size, so that tilting moments may act on the cutting blade in the groove.